Issues about engineering of the WorldTrade Center towers

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Headline: Towers' collapse shocks engineers

Source: United Press International, Tuesday, 11 September 2001

[Sorry, I don't have an URL -- this news item is reposted from Paul Milne's board]]

A lead engineer who worked on New York's World Trade Center Towers expressed shock Tuesday that the 110-story landmarks in Lower Manhattan collapsed after each tower was struck by a hijacked passenger jetliner.

Lee Robertson, the project's structural engineer, addressed the problem of terrorism on high-rises at a conference in Frankfurt, Germany, last week, Chicago engineer Joseph Burns told the Chicago Tribune.

Burns said Robertson told the conference, "I designed it for a (Boeing) 707 to hit it."

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My thoughts: In all fairness, it was the out-of-control, jet-fuel-fed fires that eventually collapsed the (no doubt weakened) structures... not the impacts per se. This is more than an academic distinction. It looked to me as though one tower (the second hit) was listing ever so slightly above the point of impact, but the other stood straight until its final "pancake" collapse. This initial survival is quite amazing considering the sheer kinetic energy of a fully loaded jumbo jet moving at, what, 200 or 300 miles per hour. (The little-known collision of a lightly loaded WW2 bomber with the Empire State Building at the end of the war was almost trivial by comparison! See the book, "The Sky is Falling")

A further point, to me it looked as though the "open" floor plan with large windows allowed a lot of the energy of the initial impacts to be dissipated outward. Bad for anyone below but perhaps this also contributed to the initial survival of the skyscrapers.

Although the innovative lattice design (new for the 1970s) eventually underwent a cascading failure (admittedly a BAD THING that designs should resist!), I offer the thought that the hour's delay before the first collapse surely saved tens of thousands of civilian lives. I think the designers ought to be proud of themselves that the towers stood at all immediately after the impacts.

Sadly, the delay also meant that hundreds of fire fighters and other rescue workers had time to go inside before the collapse occurred; last time I checked the estimate was 200 FDNY and some 70+ NYPD were unaccounted for.

I have a relative in New York City -- a great-uncle, i.e. my grandmother's brother -- who retired a few years ago after almost 50 years of being a factory rep for large valve/drain makers; he played a small part in the design and construction of the towers. Yesterday morning he was out fishing off Long Island with a friend who retired from the industry too; and a good friend of theirs was one of the lead construction engineers on one of the twin towers. My uncle's engineering comments last night were centered on the spinkler system -- he initially was dismayed the sprinklers didn't put out the fires, but after a moment's reflection he agreed with me that no reasonable building system (especially at that height above the street) could be expected to survive the impact *plus* then put out fires from tens of thousands of gallons of jet fuel.

Despite all sorts of speculation about extra bombs, etc. etc., to me as an "amateur engineer" it sure seems credible that the entire sequence of events in NYC was the result of the impacts and especially the subsequent fires.

Thoughts, anyone?

-- Andre Weltman (aweltman@state.pa.us), September 12, 2001

Answers

Ah, found this on ABCNews.com:

Headline: Final Collapse

Experts: Twin Towers Were Designed to Withstand High Impact

Source: ABC News, 11 Sept 2001

URL: http://abcnews.go.com/sections/scitech/DailyNews/WTC_towers_structure0 10911.html

They survived powerful hurricane gusts, even a bomb explosion, but this morning's two intentional plane crashes reduced the twin towers of the World Trade Center to rubble.

The 110-story towers, the tallest buildings in the city and the fifth and sixth tallest in the world, collapsed in billows of debris following two plane crashes to their sides.

By early evening, another building in the World Trade Center system, a 47-story, 570-foot tall building known as Seven World Trade Center, also collapsed after fires had raged all day in its foundation. The evacuated building was damaged when the tower above it collapsed earlier.

Despite initial damage from the crashes, the two twin towers remained standing for just over an hour this morning and appeared to be a testimony to the abilities of structural engineering.

But experts say structural damage, caused mostly by fires following the crashes, was evidently severe enough to overburden the lower sections of the towers and eventually cause both towers to topple.

"The World Trade Center was designed as a very large tube with steel columns on the perimeter of the building," said John Cryan, president of Severud Associates, a structural engineering firm that provides consulting to skyscraper architects. "What must have happened is the top part of the buildings probably collapsed and that put too much weight on the lower halves and that had a domino effect on the entire towers."

Built Like a Steel Tube

The World Trade Center hosts an estimated 50,000 employees and receives an average of 1.8 million visitors annually.

Part of the severity of the damage, Cryan believes, was the place of impact by the planes. The lower the crashes, the greater the damage to the towers' overall integrity. The first tower to crumble, the southern tower, was the one that had received the lowest strike by an oncoming aircraft.

Rich Behr, a professor of architectural engineering at Pennsylvania State University, further points out that the approximate one-hour delay in the towers' collapses suggest the main damage was likely caused, not by the plane strikes, themselves, but by fires that burned inside the buildings for more than an hour following the crashes. These fires, fueled by the aircrafts' fuel tanks, likely caused the steel beams to melt and lose their stiffness.

"It was the post impact fire that was the major culprit," said Behr. "After the impact, there was no sign of stress. [Then], after an hour of flame weakened steel and [led to the] collapse."

Built by the Port Authority of New York and New Jersey in 1973, the World Trade Center towers were the best examples of tube buildings of their time. Tube buildings are reinforced by closely spaced columns and beams in their outer walls, forming a steel tube. A series of glass windows fill in the space between the beams. And an internal core beam adds to the stability of tube structures.

To ensure the towers rested on solid bedrock and not the six acres of landfill that existed at the site of the towers, workers dug through more than 70 feet below ground before beginning construction of the twin towers in the early 1970s.

Engineers at Leslie E. Robertson, an engineering foundation based in New York City, designed the building's structure. Employees at this organization were not immediately available for comment.

Cryan explains that skyscrapers like the World Trade Center are designed to take less localized impacts on a daily basis. The steel beam-lined buildings rely on their tube network of beams to sustain hurricanes and seismic events. On Feb. 26, 1993, one of the towers even survived a bomb explosion at its base. The explosion created a 22-foot-wide, five-story-deep crater in the tower.

But today's impacts clearly exceeded the two towers' standing power. "I can't imagine the force of these crashes on the towers," said Cryan. "I can't imagine anybody wanting to do this."

-- Andre Weltman (aweltman@state.pa.us), September 12, 2001.


Headline: 'Magnitude Beyond Anything We'd Seen Before'

Towers Built to Last But Unprepared For Such an Attack

Source: Guy Gugliotta, Washington Post Staff Writer, Wednesday, September 12, 2001; Page A18

URL: http://www.washingtonpost.com/wp-dyn/articles/A14337- 2001Sep11.html

Built to withstand earthquakes and hurricane-force winds, and equipped with enhanced security after a 1993 terrorist bombing, the twin towers of New York's World Trade Center were supposed to last. Their architect boasted that they could withstand the impact of a jumbo jet.

But when two hijacked commercial jetliners crashed into the 110-story structures within 20 minutes of each other early yesterday morning, experts flinched, for "what we saw today was several orders of magnitude beyond anything we'd seen before," said the National Academy of Sciences' Richard Little, who has overseen several studies on how to protect buildings from terrorist attacks.

"We were hopeful at first," said Pennsylvania State University architectural engineer Kevin Parfitt, who teaches a course in building failures. "But the longer the fire burned, the more we feared the outcome."

With justification. In just under an hour, a raging fire from burning jet fuel softened or perhaps melted the steel strength members supporting 50 floors of undamaged skyscraper above the point of impact in the South Tower. The top floors slumped to the damaged area, and the impact of the dead weight caused the entire building to pancake to the ground. A half-hour later, the North Tower collapsed in the same way.

By late afternoon, the 47-story Building 7, another of the center's seven buildings, had also fallen after burning all day. Building 6, the U.S. Customs House, was a smoldering, soot-blackened hulk.

Experts agreed that collapse of the two towers was almost inevitable; although their "tube structure" design was their greatest source of strength, it was also an Achilles' heel. For someone who wanted to bring them down, a guided missile filled with jet fuel was perhaps the only way.

The towers were built like "rectangular doughnuts," Parfitt said. Strength came from a central steel core and from steel columns spaced closely around the perimeter of each building. There was no structural support between the core and the outer walls.

"When the planes come through, they cut through a number of those [perimeter] columns," Parfitt said. "At the same time, the planes are starting transcontinental flights, and they have full tanks of aviation fuel. You get a massive explosion and a fire."

The initial jet fuel explosions most likely blew the insulation off the towers' girders, Parfitt suggested, incinerated easy combustibles and gave the ensuing fires free access to the unguarded steel. "Sprinklers aren't going to do too much in that situation," Parfitt said.

For the people inside the buildings trying to escape, what followed was a macabre race against time, and the odds were not good. Each of the Trade Center towers had 250 elevators, but only three stairwells. Between 20,000 and 25,000 people had to get out of each building as rapidly as possible.

In 1993, after terrorists set off a bomb in a basement garage, it took four hours to evacuate the towers, but Dennis Wenger, director of the Texas A&M University Hazard Reduction and Recovery Center, said half the occupants got out in the first hour.

"There was no panic, and a lot of cooperation," even though the stairwells were "quite narrow" and smoke was wafting upward as people climbed down, said Wenger, who studied the 1993 evacuation.

In some ways, yesterday's scenario might have seemed better -- the fires were above most of the occupants. But what they didn't know was that -- in the South Tower -- they had less than an hour to get out. The North Tower was not much better.

The end came when the fire had softened the girders so that the weight above the crash sites became unsupportable. The South Tower, hit lower down, fell first beneath the greater weight. The North Tower, with less weight above the explosion, held out a bit longer: "The whole thing just imploded," said Melvyn Blum, 55, a real estate executive who was watching through a telescope from his 44th- floor office a few miles away on Manhattan's Seventh Avenue, "just like you see when they take buildings down with dynamite."

Angus Kress Gillespie, author of the 1999 book "Twin Towers: The Life of New York City's World Trade Center," said architect Minoru Yamasaki had designed the towers to withstand the impact of a jumbo jet, "but planes have become bigger" since the center was built in 1972. Minoru Yamasaki Associates (MYA) issued a statement yesterday saying the firm was in contact with authorities and had offered assistance. "We believe that any speculation regarding the specifics of these tragic events would be irresponsible," the statement said. "For obvious reasons, MYA has no further comment at this time."

By late Tuesday, few were criticizing Yamasaki for misplaced bravado. After the 1993 bombing, the center's towers "were probably among the half-dozen strongest buildings in the world, but it couldn't withstand that kind of insult," Little said.

Cesar Pelli, designer of the Petronas Towers in Kuala Lumpur, Malaysia, the world's tallest buildings, suggested that although "it will take structural engineers a long time to figure out exactly how" the towers collapsed, he agreed that "no building is prepared for this kind of stress.

"I feel a tremendous sense of loss, but this is insignificant when you think of the horror of the loss of life," Pelli said. "The grief is just unimaginable."



-- Andre Weltman (aweltman@state.pa.us), September 12, 2001.


Structural steel has a allowable stress let's say is 20000 psi at around 100 deg F. As temperature rises the allowable stress in the columns may drop to 2000 psi or less at let's say 900 deg F. These columns are supporting from the fire upward the total weight of the tower. The weight is resisted by the stress the columns can develope which is very little at say 2000 deg F. The residual stress is zero, the columns fail, and the whole upper section falls down through the lower section.

-- David Williams (DAVIDWILL@prodigy.net), September 12, 2001.

Thanks David.

So, once the steel melts/buckles, the top of the building (above the fires) sort of became a giant hammer falling one story onto the rest of the building... boom down to the next floor, and onwards to the next...

-- Andre Weltman (aweltman@state.pa.us), September 12, 2001.


I don't think the steel reached the melting point. Steel melts at about 2730 deg F. Structural steel used in buildings is coated with a fire retardant which will protect the steel from overheating in case of a fire but would not protect from an inferno. If you walk down the fire escape in a building you will see the material sprayed onto the columns. Three energy sources can be identified (1) the kinetic energy of the aircraft, (2) the burning of the fusalage, and (3) the jet fuel. The kinetic energy of the plane is 1/2 by the speed squared by the mass. A 170 ton jet with a speed of 300 mph would have an enormous amount of energy to dissipate as it entered the building. The kinetic energy was maximum at impact and zero within a few seconds. Add to this the energy contained in the jet fuel and I believe a hell on earth existed. Apparently, it took about (1) hour for the steel to reach the temperature where the residual stress was not enough to support the upper part of the building and failure occurred. I would estimate this temperature for failure at about 1200 deg. F. Probably the fuel burned at about 2000 deg F. Watching the news coverage, a structural engineer said that they had located the columns at (3)foot centers and I am only guessing at the size of the steel at this elevation but was somewhere around (3) to (4) foot thick wide flanges. In other words this building had much steel.

-- David Williams (DAVIDWILL@prodigy.net), September 13, 2001.


Headline: Believed to Be Safe, the Towers Proved Vulnerable to Jet Fuel Fire

Source: New York Times, 12 Sept 2001

URL: http://www.nytimes.com/2001/09/12/nyregion/12CENT.html? ex=1001341894&ei=1&en=86efb8f8f0fe9765

The cause of the twin collapse yesterday of the World Trade Center towers in downtown Manhattan was most likely the intense fire fed by thousands of gallons of jet fuel aboard the two jetliners that crashed into the buildings, experts on skyscraper design said.

The high temperatures, of perhaps 1,000 to 2,000 degrees, probably weakened the steel supports, the experts said, causing the external walls to buckle and allowing the floors above to fall almost straight down. That led to catastrophic failures of the rest of the buildings.

The towers were built to withstand the stresses of hurricane-force winds and to survive the heat of ordinary fires. After the 1993 trade center bombing, one of the engineers who worked on the towers' structural design in the 1960's even claimed that each one had been built to withstand the impact of a fully loaded, fully fueled Boeing 707, then the heaviest aircraft flying.

No engineer could have prepared for what happened yesterday, the experts said. "No structure could have sustained this kind of assault," said Richard M. Kielar, a spokesman for Tishman Realty and Construction Company, the construction manager for the original project.

The enormous heat from the jet fuel fire probably caused the steel trusses holding up concrete-slab floors and vertical steel columns to bend like soft plastic, said Jon Magnusson, chairman and chief executive of Skilling Ward Magnusson Barkshire in Seattle, a structural engineering firm that worked out the original design.

The skyscrapers had two means of defense against normal fire damage, Mr. Magnusson said. One, thick layers of insulation sprayed onto the steel beams, could have been breached by the initial crash, he said. Another, the building's sprinkler system, may have been disabled as well, or it may simply have been useless in the heat of the jet fuel fire.

Although they resisted collapse immediately after the planes' first impact, the hundreds of steel columns spaced around the outer facing of each tower eventually failed.

"They buckled outward and then the floors came down," said Mr. Magnusson, who warned that no conclusions could be reached yesterday since the information available was so sketchy.

Other experts agreed that the extreme conditions caused by the fire, and not unusual vulnerabilities of the buildings, were the likely causes of the collapse.

"There isn't anything particularly vulnerable about it," said Aine Brazil of Thornton-Tomasetti Engineers in New York, a structural engineering firm that worked on the Petronas Towers, the world's largest buildings, in Malaysia.

Buildings are simply not designed to withstand "the extreme levels of heat that would be found in the situation with the amount of jet fuel and the explosion that occurred," Ms. Brazil said.

Mr. Kielar, the Tishman spokesman, said it was too early to piece together a precise train of events, but he agreed that weakening by fire, followed by catastrophic collapse of the floors, was the most likely possibility. "As the structure warped and weakened at the top of each tower, it along with concrete slabs, furniture, file cabinets and other materials became an enormous consolidated weight that eventually, progressively crushed each tower below," he said in a statement.

The later collapse of the smaller 7 World Trade Center could have been caused by a combination of falling debris and a less intense fire one not accelerated by jet fuel lasting several hours, said Brian McIntyre, chief operating officer of Skilling Ward. Such a building is "basically designed to resist heat buildup for three hours," he said.

The structural design of the two towers, fairly common now, was considered innovative in its day. Instead of the heavy internal bracing and heavy exterior masonry of, for example, the Empire State Building, the designers of the trade center towers chose a light glass-and-steel facing threaded by steel columns. Those columns, 61 on each side, gave the towers most of their stiffness and largely held them up, said John Schuring, a professor and chairman of civil engineering at the New Jersey Institute of Technology.

"The major strength of the building is in its skin," Dr. Schuring said.

There was also a cluster of columns in the center, supporting structures like the stairs and elevators, he said. A network of steel trusses ran between the two sets of columns, holding up each concrete floor and providing further strength to the buildings.

A special set of plates on each floor ran among the trusses, serving to dampen stresses on the buildings caused by winds of up to 200 miles per hour, said Jack Cermak, president of Cermak Peterka Peterson in Fort Collins, Colo., the firm that did the wind-tunnel testing for the design of the towers.

Dr. Cermak agreed that the impact of the crash itself probably could not have collapsed the massively reinforced building on its own.

"I presume, without knowing the details, that that collapse was caused by weakening of the structure due to the heat," Dr. Cermak said.

Matthys Levy, an architect at Weidlinger Associates and the author of "Why Buildings Fall Down" (Norton, 1992), watched the first tower collapse while standing at Seventh Avenue and Houston Street, some 20 blocks away.

"I saw the beginning of the top moving down, and the whole thing collapsed in a cloud of smoke," Mr. Levy said. "From what I saw, it seemed to come straight down."

Mr. Levy said the situation was much different from the one that occurred in 1945 when a much smaller plane slammed into the Empire State Building. That plane, a bomber with a smaller impact and less fuel, ripped a 20-foot hole in the structure, but the building remained standing.

There was some disagreement yesterday about whether, decades later, the trade center towers had been designed to withstand an impact from an airliner filled with fuel.

The engineer who said after the 1993 bombing that the towers could withstand a Boeing 707, Leslie Robertson, was not available for comment yesterday, a partner at his Manhattan firm said.

"We're going to hold off on speaking to the media," said the partner, Rick Zottola, at Leslie E. Robertson Associates. "We'd like to reserve our first comments to our national security systems, F.B.I. and so on."

But Anthony G. Cracchiolo, director of priority capital programs for the Port Authority of New York and New Jersey, which owned the buildings, said little thought had been given to the possibility of a plane crash into the towers.

"We never were asked to consider trying to protect the building from such a threat," said Mr. Cracchiolo, who was among those who coordinated the reconstruction after the 1993 bombing. "As structural engineers, there is nothing we could have done to protect the building from a direct impact from a plane as large as these."

Melvin Schweitzer, a member of the Port Authority board of commissioners from 1993 to 1999, said, however, that the board repeatedly inquired about that possibility. "We were just told that architects had explained that the building was designed to withstand a jet," Mr. Schweitzer said. "Frankly, when we raised that question, most of us were thinking of a small plane."

The architectural firm for the trade center, Minoru Yamasaki Associates of Rochester Hills, Mich., declined to answer specific questions about the collapse, and issued only a brief statement.

"The company has been in contact with law enforcement authorities, and we will provide any assistance we can to aid the rescue efforts," the statement said. "In this time of national emergency, we believe that any speculation regarding the specifics of these tragic events would be irresponsible."



-- Andre Weltman (aweltman@state.pa.us), September 13, 2001.


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